This invention relates generally to a system and method for measuring the roughness of a surface of a substrate, and in particular, to a system and method for measuring the microroughness of a surface of a substrate.
Various types of substrates are used today that require a flat surface. For example, a flat semiconductor substrate is needed to fabricate electrical devices on the substrate so that the devices have the desired properties. A disk drive media substrate also needs to have a flat surface so that data may be magnetically or optically encoded onto the surface of the substrate. Similarly, substrates for liquid crystal flat panel displays also require flat surfaces. As smaller devices are formed on the semiconductor substrate and as more data is encoded on the disc drive media substrate, the surfaces of the respective substrates must have less roughness overall, and the average size of the roughness must be reduced.
For example, as the size of the active devices, such as transistors formed on a semiconductor substrate is reduced, it is necessary to use correspondingly thinner gate dielectrics to separate the substrate from the gate electrode and achieve the desired performance of the transistor. For current 0.18 mm transistor design rules, the gate dielectric must be only 50 angstroms thick. As a comparison, a single strand of human hair is 20,000 times wider than this gate dielectric. However, even through the thickness of these gate dielectric must be reduced, the breakdown voltage strength of these gate dielectrics must be maintained. For a gate voltage of only 2 volts, the gate dielectric must be able to withstand an electric field in excess of 4 megavolts/cm.
To meet these design requirements, it is necessary to minimize the roughness of the semiconductor substrate, especially at the interface between the gate dielectric and the substrate. The roughness at the interface between the gate dielectric and the substrate is known as microroughness because the size of the roughness is on the order of only 1 angstrom. The microroughness is usually characterized by an average size of all of the microroughness particles, known as an RMS value, and a number of microroughness particles that have a particular microroughness size, known as the size density. For a gate dielectric that is 50 angstroms thick, any roughness that is on the order of a few angstroms is not acceptable and must be removed, or the device may not meet its desired performance.
Thus, there is a need for measuring systems that effectively measure and control the microroughness of the surface of the substrate. There are two conventional methods for measuring microroughness. One is an atomic force microscopy (AFM) method in which a microscope moves an atom-sized measuring probe along the surface of the substrate, and the microroughness of the substrate is determined. This method is slow, and requires contact with the surface of the substrate, which is undesirable. Due to these limitations with this method, atomic force microscopy is not well suited for measuring the microroughness of substrates in real time in order to control a fabrication process.
Reflectance scatterometry is another conventional method that involves using a light source to reflect light off of the surface of the substrate, and measuring the scattering that occurs to the light due to the microroughness. From the scattering of the light, the microroughness may be determined. This method does not contact the surface of the substrate and is faster than atomic force microscopy, but requires a large portion of the substrate surface for testing. For real-time control of a fabrication process, only a small area of the substrate is available for testing since the gate dielectric, for example, may be only 0.5 micrometers long. In addition, this method does not have the range of sensitivity necessary to measure microroughness down to the angstrom size roughness level, which is needed for current fabrication processes.
Thus, there is a need for a system and method for real time monitoring and measurement of the microroughness of a surface of a substrate to control a fabrication process which avoid these and other problems of known devices, and it is to this end that the present invention is directed.